As part of my bushcraft course we are doing a "40L challenge" – ie we can only pack 40L of stuff for the whole weekend.

In preparation , I thought I'd work out how many calories I got through this weekend (also bushcrafting) so I can try and make sure I don't end up ravenous. I'm already at 3876 and haven't added cups of tea or marg on bread/ potatoes yet . . . . I'm gonna need a bigger rucksack!

I've done a weekend with a 30L pack, and I'm sure others have done it with less.

What is your gear list?

Oh, and are you sure your pack is actually 40L? Often packs differ from their stated volume. For example, osprey might call a pack the XX40, but the small size might only be 38L, the medium 40L, and the large 42L.

If you use only hard objects to measure volume you will under-measure the pack: You'll measure a 40-liter pack as 35-ish liters. Or less

If you use 40 liters of loose lentils or rice, you'll get a very accurate volume, but that's a LOT of lentils (10.5 gallons).

If you combine the two techniques – rigid containers like 2-liter soda bottle, gallon milk jugs, and assorted Nalgene bottles with a few gallons or lentils, gravel or rice, you'll get an accurate measurement but minimize the amount of rice/lentils you use.

If you plan on doing this a lot, make some very floppy beanbags to hold the rice/lentils in 1-liter and 2-liter quantities to fill in the gaps around rigid containers without making a mess. Stitched out of cloth is sturdy and very flexible. A liter of rice in a gallon ziplock (burped of air) is quick and easy.

Or coconut oil/olive oil @ ~240-50 cal/oz. Also, there are many other high calorie nut butters available that provide a lot of calories in a very small amount of space; walnut, macadamia, cashew, and pecan come to mind.

Another approach would be to bring a relatively small amount of some high carb powder like maltodextrin, say 15-20 ounces, and use it to support the metabolism of body fat. If you're only out for a weekend, this would be a very effective way to cut down on the space required for food in your pack.

Fat is actually the most satiating type of food. You will find your body will want to eat less of it. You will feel full more quickly. If you are used to eating high-carb, it will take you about a week or so for your body to get used to eating fats. You may experience stomach upset/diarrhea. If you do, up your fat intake slowly. A great way to start the switch is "Bulletproof Coffee." Your regular morning coffee with 2-4 Tb. of butter melted into it, hold the sugar. That's been my sole breakfast for a couple of months now, and it's great (although my days are pretty sedentary, it lasts me 3-4 hrs. 16oz coffee & 3T butter).

Yes, you can use coconut oil in most butter recipes – as long as your outside temps are below say 76*. There are also vegan margarines as well.
Frankly though, you can use olive oil in most recipes for hiking as well.

"Another approach would be to bring a relatively small amount of some high carb powder like maltodextrin, say 15-20 ounces, and use it to support the metabolism of body fat. If you're only out for a weekend, this would be a very effective way to cut down on the space required for food in your pack.".

Agree with this approach. I started off my UL backpacking thinking it was all about finding those higher calorie per ounce foods. I have since learned for short duration trips that I can carry a much lighter food load AND have more energy by carrying high carb foods and maximizing burning fat reserves.

I find that a day's normal food for walking takes up about 1.5 litres of pack capacity (pasta, biscuits, muesli, cheese etc) thus 2 days = 3 litres. What are you putting in the other 37 litres? I very comfortably get gear and 8 days food into a 40 litre pack for conditions around freezing.

I agree with mark Your problem is in your gear not your food. My gorilla only holds 46 liters and is never full when I go back packing in the summer. I fit my whole set up and an Ipad(movies, books and on sight photo editing) and a DSLR camera.

If the weather is good enough try an emergency bivy with full clothes on a pad. Ive used that set up before with no issues. but you might want to test that at home first to make sure.